As above, if a player is using the Twilight Aura in a modded biome, it tries to find the colour information for the firefly particles. As these are modded biomes, it finds none, and causes a hard crash. This happens for any players observing these particle effects, including in multiplayer.

[01:07:46] [Render thread/ERROR]: Reported exception thrown!
net.minecraft.class_148: Exception while adding particle
at Not Enough Crashes deobfuscated stack trace.(1.19+build.4) ~[?:?]
at net.minecraft.world.World.tickEntity(World:473) ~[?:?]
at net.minecraft.client.world.ClientWorld.method_32124(ClientWorld:250) ~[?:?]
at net.minecraft.world.EntityList.forEach(EntityList:54) ~[?:?]
at net.minecraft.client.world.ClientWorld.tickEntities(ClientWorld:246) ~[?:?]
at net.minecraft.client.MinecraftClient.tick(MinecraftClient:1813) ~[?:?]
at net.minecraft.client.MinecraftClient.render(MinecraftClient:1101) ~[?:?]
at net.minecraft.client.MinecraftClient.run(MinecraftClient:734) ~[?:?]
at net.minecraft.client.main.Main.main(Main:237) [client-intermediary.jar:?]
at net.fabricmc.loader.impl.game.minecraft.MinecraftGameProvider.launch(MinecraftGameProvider.java:461) [fabric-loader-0.14.9.jar:?]
at net.fabricmc.loader.impl.launch.knot.Knot.launch(Knot.java:74) [fabric-loader-0.14.9.jar:?]
at net.fabricmc.loader.impl.launch.knot.KnotClient.main(KnotClient.java:23) [fabric-loader-0.14.9.jar:?]
Caused by: java.lang.NullPointerException: Cannot invoke "ladysnake.illuminations.client.data.BiomeSettings.fireflyColor()" because the return value of "ladysnake.illuminations.client.config.Config.getBiomeSettings(net.minecraft.class_2960)" is null
at ladysnake.illuminations.client.particle.FireflyParticle.(FireflyParticle.java:55) ~[illuminations-1.10.8.jar:?]
at ladysnake.illuminations.client.particle.aura.TwilightFireflyParticle.(TwilightFireflyParticle.java:27) ~[illuminations-1.10.8.jar:?]
at ladysnake.illuminations.client.particle.aura.TwilightFireflyParticle$DefaultFactory.createParticle(TwilightFireflyParticle.java:186) ~[illuminations-1.10.8.jar:?]
at ladysnake.illuminations.client.particle.aura.TwilightFireflyParticle$DefaultFactory.createParticle(TwilightFireflyParticle.java:177) ~[illuminations-1.10.8.jar:?]
at net.minecraft.client.particle.ParticleManager.createParticle(ParticleManager:310) ~[?:?]
at net.minecraft.client.particle.ParticleManager.addParticle(ParticleManager:295) ~[?:?]
at net.minecraft.client.render.WorldRenderer.spawnParticle(WorldRenderer:2762) ~[?:?]
at net.minecraft.client.render.WorldRenderer.addParticle(WorldRenderer:2730) ~[?:?]
at net.minecraft.client.render.WorldRenderer.addParticle(WorldRenderer:2725) ~[?:?]
at net.minecraft.client.world.ClientWorld.addParticle(ClientWorld:607) ~[?:?]
at net.minecraft.entity.player.PlayerEntity.handler$clf004$tick(PlayerEntity:9872) ~[?:?]
at net.minecraft.entity.player.PlayerEntity.tick(PlayerEntity:333) ~[?:?]
at net.minecraft.client.network.OtherClientPlayerEntity.tick(OtherClientPlayerEntity:38) ~[?:?]
at net.minecraft.client.world.ClientWorld.tickEntity(ClientWorld:267) ~[?:?]
at net.minecraft.world.World.tickEntity(World:466) ~[?:?]
... 10 more